Filament heating transformer having a variable output voltage



y 1952 G. USSELMAN 2,598,321

' FILAMENT HE NG TRANSFORMER HAVING A VARIABLE OUTPUT VOLTAGE Filed May 21, 1948 2 SHEETS-4815314 INVENTOR AT ORNEY May 27, 1952 G. 1.. USSELMAN 2,598,321

FILAMENT HEATING TRANSFORMER HAVING A VARIABLE OUTPUT VOLTAGE Filed May 21, 1948 2 SHEETS-SHEET 2 INVENTOR ATT RNEY Patented May 27, 1 952 UNITED STATE FILAMENT HEATING TRANsFonM-EnHAv- ING AVARIABLE OUTPUT VOLTAGE George L. Usselman, Port Jefferson, N. Y.-, as signor to Radio Corporation of America; a'corporation of Delaware Application May- 21, 1948, serial Nmzsaw This invention relates to transformers in general, and in particular to a novel method of varying the power output of certain kinds of transformers.

The purpose of this invention is to provide an improved filament transformer having means to change the output voltage by the movement of an iron core shunt.

In prior art transformers it is customary to vary the taps of the transformer windings by a multiple contact switch or to use extra bucking or boosting windings with movable iron cores to change the transformer output voltage.

The present invention employs a simple flux shunting core method of changing the transformer output voltage by movement of the core.

This invention is best suited for application to transformers that inherently have high flux leakage which must be tolerated in order to gain high voltage insulation between the secondary winding and other elements of the transformer; or in order that the secondary winding shall have low electrostatic capacity to other parts of the transformer for radio frequency use. Filament transformers for rectifiers and for radio transmitting tubes are examples.

This invention will be more clearly understood by referring to the accompanying drawings, wherein:

Fig. 1 is a side view of the transformer of this invention;

Fig. 2 is an end view of Fig. 1; and

Fig. 3 is a side view of another embodiment of this invention.

Referring now in detail to Figs. 1 and 2 of the drawings, transformer core I has primary coils 2 and 2' surrounding the core on two legs and a secondary coil 3 surrounding the core on a third leg. A movable yoke or U-shaped core shunt member 4 is provided adjacent the secondary coil 3. The legs of the U-shaped core 4 are arranged each side of secondary coil 3. The yoke 4 is pivoted at AA by means of a shaft 9 which is arranged to be rotated by means of motor 5, worm 6, and worm wheel I. The shaft 9 rotates in bearings 8 and is secured to core 4 by means of supporting brackets l0. Yoke 4 is preferably made of laminated iron, or of about the same material as the core I. The primary and secondary windings are of ordinary transformer construction, containing sufficient turns to give the desirable step-up or step-down ratio, and are provided with sufficient insulation to withstand the required voltage.

In the operation of the transformer of this in- 2 Claims. (01. 17 1 119) vention, when the secondary winding offa' trans former is loaded it offers considerable impedance to the flow of magnetic flux through the section of the transformer core which it encircles. This causes some of the flux generated by the primary Winding to flow outside of the secondary winding, which flux is called leakage flux. If a low reluctance flux path is provided around the secondary winding much less flux will flow through the section of core enclosed by the secondary winding and correspondingly more flux flows through the shunt. Consequently the voltage on the secondary coil will be greatly reduced. However. this large voltage reduction depends upon the secondary winding being loaded.

In this invention, the means for varying the flux shunting effect operates by lifting the yoke :i by rotating it on shaft 9, thereby changing the air gap between the yoke and the transformer core. With the yoke down against the transformer core (Fig. 1), the output power will be low. With the yoke 4 lifted furthest away from the core, the output power of the transformer will be maximum.

The transformer of this invention has two out standing advantages. When the yoke i down so as to provide maximum flux leakage, the load, such as tube filaments, may be energized at low power even when full primary voltage is applied. i'hen, to fully energize the filaments, the yoke 4 is raised to its maximum position.

The yoke 4 also may be used to adjust the filament voltage by adjusting the amount of flux leakage. This may be done by raising or lowering the yoke. Another method of operation would be to set the yoke at some desired position for a certain output. Then when full primary voltage is applied, the filament of the tube being cold at the start, will have low resistance at the start. but it is prevented from drawing abnormally high current because the larger part of the flux will be shunted. As the filaments heat up, the current tends to fall but less flux leakage occurs, which tends to maintain the filament current. In other words, the very large initial rush of filament current (which is likely to damage the tube) is prevented in this case.

In the embodiment shown in Fig. 3 of the drawings, the core I, primary 2, and secondary 3 are of substantially the same construction as that shown in Figs. 1 and 2. The yoke 4|, however, is of slightly different construction in that the end which forms an air gap between one of its faces and the secondary leg of the core is cut at an angle of approximately 45. The far end of yoke 3 4| is provided with a limit switch pin l3 for actuation of the limit switches I l and 12 which are connected in the motor circuit to prevent an over travel of yoke 4| which might harm the mechanism.

In other forms of this invention, instead of a worm andgear-arrangement to operate-the yoke, a cam arrangement could'be devised'which would apply maximum force or leverage at the position where the air gap between the yoke and the core is smallest. This is the position where the magnetic force on the yoke is greatest. The region of the small air gap is the position wheremaximum voltage control is obtained.

One example of the transformer ofthisiinvention which was constructed using a 1.7 kilowatt filament transformer for a radio transmitter, having a high flux leakage secondary winding for low electrostatic capacity to other transformer parts, was found to give satisfactory results where there was asufficient load on the filaments of :the tubes.

:Manualcontrol of the shunt may be used instead of control by motor. This would bedone by replacing the motor with .a handwheel orby a 1:

set of shafts-and gears leadingto a crank or dial on the front'panel of the appaartus.

What is claimed is:

1. An'adjustable voltage transformer, comprising a-ferromagnetic-core, a primary winding surroundinga portion of said core, a secondary winding surrounding another portion of said core and having its axis extending at right angles to the axis of the primary winding, a pivotallymounted ferromagnetic yoke member positioned, with respect to said secondary winding and said core, so as to shunt magnetic flux around the secondary winding, and a motor-operattedmechanism mechanically coupled to said yoke for moving said yoke about its pivot to change the air gap between said core and said yoke.

2. A transformer in accordance with claim 1, wherein the yoke member is U-shaped.

GEORGE L. USSELMAN.

REFERENCES CITED The'followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Name "Date 862,361 Thompson Aug. 6,1907 1,174,053 Burns Mar. 7, 1916 1,378,151 Thordarson May 1'7, 1921 1,683,389 Kaehni et'al Sept. 4,1928

FOREIGN PATENTS Number Country .Date

566,708 France .Feb. 19, 1924 

